<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        arr:hover {
            background: red;
        }
    </style>
</head>
<body>
    <script>
    // var arr=[1,2,3,4,5,6,7,8,9];
    // //在页面上生成一个ul，ul中的li元素，由arr数组元素生成
    // //最终反馈到页面中
    // //join
    // //1+2+3+4+5+6+7+8+9
    
    // var str=arr.join('</li><li>')
    // document.write('<ul><li>'+str+'</li></ul>')
    // console.log(str);    

   

    // var arr=['tom','john','张三'];
    // var str=arr.join('</li><li>')
    // document.write('<ul><li>'+str+'</li><ul>')
    // console.log(str);

    var arr=[1,2,3,4,5];
    arr.reverse()
    for (var i=0;i<arr.length;i++){
        console.log(arr[i])
    }
    var arr1=[1,2,3,4]
    var arr2=[5,6,7]

    console.log(arr1.concat(arr2))


    var emptyArr=[]
    for (var i=0;i<5;i++){
        var childArr=[]
        for(var j=0;j<5;j++){
        childArr.push(j)  
        }
        console.log(childArr)
        emptyArr.push([childArr])
    }
    console.log(emptyArr)


    
    </script>
</body>
</html>